1. Field of the Invention
The present invention relates to signal attenuation circuits, and in particular, to digitally controlled signal attenuation circuits.
2. Description of the Related Art
Digitally controlled attenuator circuits are well-known in the art. Such attenuator circuits are generally used in controlled impedance environments, and allow the attenuation to be controlled in units or fractions of decibels (dB). One particular type of such attenuator is referred to as a linear-in-dB attenuator, in which a thermometer code type of switching, or control, signal causes the attenuation to vary in single dB steps.
Referring to FIG. 1, a conventional digitally controlled linear-in-dB attenuator includes a resistive ladder circuit with series resistances Rs2-Rs7 and shunt resistances Rp1-Rp7, interconnected substantially as shown, to which the input voltage signal Vin is applied. The voltages at nodes N1-N7 are applied to the throw electrodes of the single-pole, single-throw switch circuits S1-S7. The pole electrodes of these switches S1-S7 are mutually connected to provide the output signal Vout. The switches S1-S7 are controlled with a thermometer code control signal to selectively close the individual switches, depending upon the desired attenuation. (As one example embodiment, the series resistances Rs2-Rs7 would have nominal resistance values of 109 ohms, while the shunt resistances Rp1-Rp7 would have nominal resistances of 8170 ohms.)
Referring to FIG. 1A, a problem with such conventional attenuator circuits is the limited bandwidth caused by the circuit topology. As seen in FIG. 1A, at or near a certain frequency Fc, the attenuation is no longer constant and begins to increase. This is due to the switch circuits S1-S7, which are typically implemented using metal oxide semiconductor field effect transistor (MOSFET) switches with low turn-on resistances. As is well-known in the art, such devices typically have relatively high parasitic capacitances at their drain and source electrodes. It is this parasitic capacitance that causes the bandwidth to be limited, thereby causing the attenuation characteristics to no longer be constant above a certain frequency Fc. Further, also as shown in FIG. 1A, the bandwidth decreases as the attenuation increases. This is caused by the increased capacitance due to more of the switches S1-S7 being in their off states.